The present invention relates generally to automotive HVAC systems for controlling the environment of an automobile passenger compartment. More particularly, the invention relates to a heating control system for controlling the heating operation in a reversible HVAC system for automobiles.
This application is related to co-pending applications titled Refrigerant Flow Management Center For Automobiles, Reversible Air Conditioning And Heat Pump HVAC System For Electric Vehicles, Controller For Reversible Air Conditioning And Heat Pump HVAC System For Electric Vehicles, Anti-Fog Controller For Reversible Air Conditioning And Heat Pump HVAC System For Electric Vehicles, Air Handling Controller For Hvac System For Electric Vehicles, and System For Cooling Electric Vehicle Batteries. Each of these applications is incorporated by reference into the present application.
Automotive heating, ventilation and air conditioning, HVAC, systems have traditionally been single loop designs in which the full volume of refrigerant flows through each component in the system. In an HVAC system, refrigerant in the vapor phase is pressurized by a compressor or pump. The pressurized refrigerant flows through a condenser which is typically configured as a long serpentine coil. As refrigerant flows through the condenser heat energy stored in the refrigerant is radiated to the external environment resulting in the refrigerant transitioning to a liquid phase. The liquefied refrigerant flows from the condenser to an expansion valve located prior to an evaporator. As the liquid flows through the expansion valve it is converted from a high pressure, high temperature liquid to a low pressure, low temperature spray allowing it to absorb heat. The refrigerant flows through the evaporator absorbing heat from the air that is blown through the evaporator fins. When a sufficient amount of heat is absorbed the refrigerant transitions to the vapor phase. Any further heat that is absorbed raises the vaporized refrigerant into the superheated temperature range where the temperature of the refrigerant increases beyond the saturation temperature. The superheated refrigerant flows from the outlet of the evaporator to the compressor where the cycle repeats. Generally, the refrigerant flowing into the compressor should be in the vapor phase to maximize pumping efficiency. The operation of the refrigerant loop in conventional automotive HVAC systems is controlled by cycling the compressor on and off, and by varying the volume of refrigerant that is permitted to flow through the expansion valve. Increasing the volume of refrigerant that flows through the valve lengthens the distance traversed by the liquid before it changes to the vapor phase, allowing the heat exchanger to operate at maximum efficiency.
Advances in automotive HVAC systems have led to zone temperature control systems wherein different zones of an automobile are independently controlled. Zone control systems generally include an evaporator and expansion valve for each zone. The refrigerant flows through a compressor and condenser, then is split by a system of valves before flowing to the expansion valve and evaporator of each zone. The refrigerant flowing out of the evaporator of each zone is then recombined before returning to the compressor. A complex series of valves and plumbing is generally required to maintain a balanced HVAC system that provides individualized control for each of the zones.
With the advent of electric vehicles reversible heat pump systems have been introduced into automobiles. In a reversible heat pump system the HVAC system can either heat or cool a compartment depending on the direction of the refrigerant flow. In the air conditioning mode refrigerant flows from the compressor through an outside coil (condenser) and into an expansion valve and inside coil (evaporator) before returning to the compressor. Heat energy is extracted from air that is blown through the inside coil (evaporator) into the passenger compartment thus providing cooled air. In the heating mode a four way valve reverses the flow of refrigerant through the coils, thereby reversing the function of the coils. Refrigerant flows from the compressor through the inside coil (condenser) then into an expansion valve and the outside coil (evaporator) before returning to the compressor. Heat energy in the liquefied refrigerant flowing through the inside coil is absorbed by air that is blown through the inside coil into the passenger compartment thus providing heated air.
However, the amount of heat that can be transferred from the evaporator in an HVAC system to the condenser is limited by the system components as well as the temperature of the outside air. Therefore, under some temperature conditions a heat pump system will not be able to provide sufficient heat to warm the passenger compartment to the desired temperature. Under these conditions another heater such as an electric heater is required to provide the desired heating. A drawback of electric heaters is that they are less energy efficient than a heat pump. To supply heat to the passenger compartment electric heaters such as PTC heaters convert energy drawn from the vehicle electrical system. Therefore, energy directed towards heating reduces the energy available for propulsion of the vehicle thereby reducing the overall efficiency. Whereas a heat pump uses a small amount of energy to drive a compressor which extracts heat energy from the external environment and transfers it into the passenger compartment. A heat pump uses about 40% of the energy that a PTC heater would use to heat the passenger compartment to a desired temperature. Generally, conventional systems in electric vehicles first attempt to use the heat pump to heat the passenger compartment, then switch to an electric heater if the heat pump is not capable of supplying the required heat. Heating the passenger compartment causes a significant energy load on a vehicle, especially so when electric heaters are used.
Additionally, during air conditioning mode in conventional electric vehicle systems the air is generally cooled to a temperature slightly cooler than the desired temperature and then an electric heater is used to raise the air temperature to the desired temperature. This scheme is employed to correct for deficiencies in controlling compressors and sensing air temperature. The electric heater is employed to provide the fine control over air temperature that passengers desire. With electric compressors a temperature probe is generally used for sensing air temperature which the system attempts to control. Temperature probes have a slow response and tend to provide an inaccurate output due to location sensitivity resulting from variations in airflow. Electric heaters that are used in combination with electric compressors improve the response time of the system to changing conditions and provide a more accurate output temperature. However, the improved response time and temperature accuracy are obtained at the expense of wasted energy. Energy is wasted by initially cooling the passenger compartment to a lower than desired temperature, and then additional energy is wasted by using electric heaters to raise the air temperature.
In a conventional electric vehicle system the extra energy that an electric heater requires reduces the fuel mileage of the vehicle leading to higher operating costs and the nuisance of more frequent stops to refuel. Energy diverted to the electric heaters during either the heating mode or the cooling mode reduces the operating range of the vehicle, increases operating costs, and reduces the lifetime of the vehicle batteries by subjecting them to more discharge cycles.
One object of the present invention is to provide a system which minimizes energy consumption during a heating operation of an automotive HVAC system.
Another object of the present invention is to disclose a heating mode selection method in which the heating mode selection is dynamically updated as operating conditions change.
It is an additional object of the present invention to provide a system that increases the energy efficiency of an electric vehicle.
A further object of the present invention is to provide an energy efficient method for controlling the passenger compartment temperature of an electric vehicle.
Accordingly, the invention provides a reversible HVAC system for heating a passenger compartment of a motor vehicle. The HVAC system includes a heat pump and an electric heater for supplying heat directly to air blown into the passenger compartment. A controller initially selects the heat pump to provide the heat that is directed into the passenger compartment. The heat pump attempts for a predetermined period of time to heat the passenger compartment to a target temperature. The controller stores a heat pump gain value representing the heating capacity of the heat pump before switching the heating operation to the electric heater. During operation of the electric heater, the controller calculates a system heat gain value representing the required heating capacity to maintain the passenger compartment at the target temperature. While the electric heater is operating, the controller continuously modifies the heat pump gain value to reflect changes in the ambient temperature. When the heat pump gain value approximately equals or exceeds the system gain value the controller selects the heat pump to provide heating.
The above described device is only an example. Devices in accordance with the present invention may be implemented in a variety of ways.